As the demand for water has grown, industry has long sought processes for the desalination of salt water, such as seawater or brackish water. Two such processes are distillation and membrane processes. Membrane processes include reverse osmosis (RO), nanofiltration (NF), and electrodialysis (ED).
For membrane based methods, increased production rates are typically sought by developing new and improved membranes or by improving recovery of energy in existing desalination processes. Water recovery is a major economic parameter of drinking water production. However, this parameter is typically limited in existing processes. In addition to limited water recovery, another drawback is that these processes are typically considered energy intensive. Membrane fouling and scaling in pressure-driven membrane processes (e.g., in RO and NF) are often a major area of concern, as they can increase the cost of operating and maintaining the systems. In addition, because most membranes processes rely on an osmotic draw solution, increasing salt content typically reduces the flux of product water in existing processes due to the lower osmotic potential difference at higher salt concentrations.
Membrane distillation (MD) is a membrane process that has been investigated in small-scale laboratory studies. Mass transfer in MD is typically controlled by the vapor pressure difference across a microporous hydrophobic membrane—vapor, rather than liquid, diffuses through the pores of the membrane and the vapor condensate is the product water. MD technology has not commonly been used for commercial desalination, however, because of the low fluxes of water through the membrane typically achieved.